The mammalian nervous system does not regenerate after injury despite the fact that there are many molecules present which encourage/promote axonal (nerve) growth. There are at least three factors that are responsible for this lack of regeneration: the formation of a glial scar, the presence of inhibitors of regeneration in myelin, and the intrinsic growth capacity of adult axons. In situations involving injury, the glial scar takes some time after injury to form. It would be advantageous to encourage axonal growth during this “window-of-opportunity”, before the scar forms. It would also be desirable to be able to encourage axonal growth irrespective of scarring, e.g., for treating or preventing neural degeneration or damage associated with a disorder, disease or condition. Blocking the function of the inhibitors of regeneration present in myelin can be achieved by, e.g., neutralizing the inhibitors or altering the growth capacity of the axon so that it no longer responds to the inhibitors.
To date, three inhibitors have been identified in myelin: myelin-associated glycoprotein (MAG) (McKerracher, L. et al., Neuron, 13, pp. 805-811 (1994); Mukhopadhyay, G. et al., Neuron, 13, pp. 757-767 (1994); U.S. Pat. No. 5,932,542; U.S. Pat. No. 6,203,792; and U.S. Pat. No. 6,399,577; and WO 97/01352), Nogo (Chen, M. S. et al., Nature, 403, pp. 434-439 (2000); Grandpre, T. et al., Nature, 403, pp. 439-444 (2000)); and oligodendrocyte myelin glycoprotein (OMgp) (Wang, K. C. et al., Nature, 417, pp. 941-944 (2002). Interestingly, all three of these inhibitors bind to the same receptor to exert their inhibitory effects (Wang et al., supra; Domeniconi, M. et al., Neuron, 35, pp. 283-290 (2002); Fournier, A. E. et al., Nature, 409, pp. 341-346 (2001); Liu, B. P. et al., Science, 297, pp. 1190-1193 (2002)). Because this receptor was first identified as being a receptor for Nogo-66 ligand (a 66 amino acid extracellular domain shared by different isoforms of Nogo), it is referred to as the Nogo-66 receptor (“NgR”)(Foumier, A. E. et al., supra).
One way to neutralize an inhibitor of neural growth and regeneration is to interfere with its ability to bind to or to activate signaling by its cognate receptor. Hence, it would be useful to design molecules capable of interfering with the ability of endogenous MAG to bind to or to activate NgR. MAG derivatives, for example, which can compete with endogenous MAG for neuron binding but which cannot bind to or activate signaling by NgR would be desirable.
Our previous studies demonstrated that the ability of MAG to inhibit neurite outgrowth is distinct from its ability to bind to neurons. (See e.g., U.S. Pat. Nos. 5,932,542; 6,203,792; and 6,399,577). These studies identified one such desirable inhibitor, MAG(d1-3)-Fc, and showed that sequences in the fourth and fifth Ig-like domains or the junction between the third and fourth Ig-like domains of MAG are responsible for MAG's ability to inhibit neurite outgrowth. It would be useful to identify other molecules that can block the inhibitory effects of myelin on neural growth and regeneration.
It appears that there is overlap in the binding sites on NgR for the three ligands (MAG, Nogo-66 and OMgp) (Domeniconi et al., supra; Liu et al., supra; Wang et al. supra) as binding of one ligand is able to compete with and thus reduce binding of other ligands (Domeniconi et al., supra; Wang et al. supra). Thus, if a ligand binding site on NgR were to be blocked, the inhibitory effects of all three of the inhibitors found in myelin would likely be blocked. Furthermore, it is likely that peptide fragments derived from any one NgR ligand would block the ability of any or all of these inhibitors to bind to and activate NgR. Because these are the only inhibitors in myelin identified to date, blocking the ability of these ligands to bind to NgR and thus blocking the downstream effects of NgR signaling will likely prevent the majority, if not all, of the inhibitory effects of myelin on neural growth and regeneration.